Measurement of low-ppm mixing ratios of water vapor in the upper troposphere and lower stratosphere using chemical ionization mass spectrometry

Abstract

A chemical ionization mass spectrometer (CIMS) instrument has been developed for the fast, precise, and accurate measurement of water vapor (H₂O) at low mixing ratios in the upper troposphere and lower stratosphere (UT/LS). A low-pressure flow of sample air passes through an ionization volume containing an α-particle radiation source, resulting in a cascade of ion-molecule reactions that produce hydronium ions (H₃O⁺) from ambient H₂O. The production of H₃O⁺ ions from ambient H₂O depends on pressure and flow through the ion source, which were tightly controlled in order to maintain the measurement sensitivity independent of changes in the airborne sampling environment. The instrument was calibrated every 45 min in flight by introducing a series of H₂O mixing ratios between 0.5 and 153 parts per million (ppm, 10^−6 mol mol⁻¹) generated by Pt-catalyzed oxidation of H₂ standards while overflowing the inlet with dry synthetic air. The CIMS H₂O instrument was deployed in an unpressurized payload area aboard the NASA WB-57F high-altitude research aircraft during the Mid-latitude Airborne Cirrus Properties Experiment (MACPEX) mission in March and April 2011. The instrument performed successfully during seven flights, measuring H₂O mixing ratios below 5 ppm in the lower stratosphere at altitudes up to 17.7 km, and as low as 3.5 ppm near the tropopause. Data were acquired at 10 Hz and reported as 1 s averages. In-flight calibrations demonstrated a typical sensitivity of 2000 Hz ppm⁻¹ at 3 ppm with a signal to noise ratio (2 σ, 1 s) greater than 32. The total measurement uncertainty was 9 to 11%, derived from the uncertainty in the in situ calibrations

The airborne mass spectrometer AIMS – Part 1: AIMS-H<sub>2</sub>O for UTLS water vapor measurements

“…For the H 2 O mode of AIMS, the ion-molecule reactions in the ion source are very similar to the reactions in the D region of the ionosphere (Thornberry et al, 2013). These reactions are described by, e.g., Fite (1969), Fehsenfeld et al (1971) and Ferguson (1974).…”

“…The airborne mass spectrometer AIMS -Part 1 characterization of ambient air (Thornberry et al, 2013). In contrast to the other two techniques, CIMS and PTR-MS utilize transfer reactions between artificially produced reagent ions and ambient air molecules.…”

“…This technique was applied in flight for the first time during the MACPEX mission in 2011 (Thornberry et al, 2013). A mixture of (420 ± 8.4) ppmv H 2 in synthetic air (Airliquide GmbH) is stored in a 150 mL stainless steel cylinder (Swagelok) with a maximum pressure of 150 bar (Fig.…”

Intercomparison of mid-latitude tropospheric and lower stratospheric water vapor measurements and comparison to ECMWF humidity data

“…The Atmospheric Ionization Mass Spectrometer for water vapor (AIMS) is a linear quadrupole mass spectrometer designed 20 to measure low water vapor mixing ratios typical for the upper troposphere and lower stratosphere Thornberry et al, 2013), and, in a different configuration, HCl, HNO 3 and SO 2 . The instrument samples gas phase water vapor through a backward facing heated inlet.…”

“…As a consequence, novel concepts and instruments (e.g. Thornberry et al, 2013;Kaufmann et al, 2014;Kaufmann et al, 2016;Buchholz et al, 2017) and improved techniques for inflight (Rollins et al, 2011) and ground calibration (Meyer et al, 2015) were developed to improve the accuracy of H 2 O measurements.…”